SAN FRANCISCO -- Another portable toilet was set ablaze in San Francisco's Russian Hill neighborhood early Wednesday, at least the 18th such incident in the area since November.

The latest incident was reported at about 4 a.m. on Washington Street between Taylor and Jones streets, according to a fire dispatcher.

Firefighters responded to the area and put out the fire within minutes, the dispatcher said.

Fire Lt. Mindy Talmadge said last week that no one has seen anyone starting the fires.

In early December the fire department had to warn construction site managers about the fires and asked them to move their toilets away from buildings and trees.

The petroleum-based outhouses, commonly called porta-potties, are highly flammable and can turn into a big puddle of blue goop within minutes, scorching anything in its way, a porta-potty salesman said.

"Catch on fire? No problem. Light up the toilet paper and that'll start it. Boom. They'll be down in five minutes," salesman Cameron Vigil, of Far West Sanitation & Storage, said last week.

Vigil, who rents the toilets to sites across the Bay Area, said his company has lost at least one or two to the Russian Hill fiasco, and typically it will cost a client up to $1,000 to replace.

An arsonist has struck at least four times in November, 10 in December and four times this year. Typically the toilets are lit between 1 a.m. and 6:30 a.m. but the fires have also happened at more random times such as Saturday, Dec. 13, at 8:55 p.m.

An arsonist also decided to ring in the New Year shortly after 5 a.m.

Vigil said porta-potty fires are not uncommon but this many in such a concentrated area is unusual.